Since the traffic volume of trunk communication systems has been drastically increasing due to widespread use of the Internet, there are great hopes that a high speed optical communication system in excess of 40 Gbps will be practically implemented.
In recent years, as a technology to realize an ultra-high speed optical communication system, optical phase modulation systems have been attracting much attention. In optical phase modulation systems, data modulation is not performed to an optical intensity of a transmission laser light like a related optical intensity modulation method, but data modulation is performed to a phase of a transmission laser light. As optical phase modulation methods, methods such as QPSK and 8PSK are known. QPSK is an abbreviation for Quadrature Phase Shift Keying, and 8PSK is that for 8 Phase Shift Keying.
The advantage of optical phase modulation methods is as follows. That is, by allocating a plurality of bits to one symbol, it is possible to reduce a symbol rate (baud rate). Accordingly, an operation speed of electrical devices can be reduced. As a result, cutting of device production costs can be expected. For example, if QPSK is used, 2 bits (00, 01, 11 and 10, for example) are allocated to each of four optical phases (45, 135, 225, and 315 degrees, for example). Therefore, the symbol rate in QPSK can be reduced to ½ of the symbol rate (that is, bit rate) in optical intensity modulation methods.
FIG. 1 is a diagram called a constellation of QPSK in which four symbols of QPSK and bit sequences allocated to each symbol are illustrated on a phase plane. Correlating a bit sequence to each symbol in the optical phase modulation system is called symbol mapping. Although the case will be described below where QPSK is used as an optical phase modulation system, other optical phase modulation systems are also applicable.
In order to receive a signal light modulated with optical phase modulation, the signal light and a laser light (which is called a local light) which has the almost same optical frequency as that of the signal light are coupled by an optical element called a 90-degree hybrid. Then, the output of the 90-degree hybrid is received by an optical detector, and is converted to an electrical signal. A system like this is called an optical coherent system.
It is assumed for simplicity that each polarization state of the signal light and the local light is an identical linear polarization. If the optical coherent system is used, an alternating current component of an electrical signal outputted from the optical detector is a beat signal composed of the signal light and the local light. The amplitude of the beat signal is proportional to amplitude of the signal light and the local light, and the phase of the beat signal is equal to the difference in the optical phase between the signal light and the local light if a carrier wave frequency of the light signal and an optical frequency of the local light are identical. If the optical phase of the local light is identical with the optical phase of the laser light inputted into an optical modulator in a transmitting end, the phase of the beat signal is the optical phase applied to the laser light by the transmitting end. Therefore, the transmitted data are demodulated by transforming the phase of the beat signal into a bit sequence using symbol mapping. That is, if an optical signal with the constellation in FIG. 1 is transmitted from the optical transmitter, an optical receiver is able to receive a signal with the similar constellation.
However, in general, the value of the carrier wave frequency of the signal light does not completely coincide with that of the optical frequency of the local light. Moreover, the optical phase of the local light in the receiver does not coincide with that of the laser light inputted into the optical modulator in the optical transmitter.
The optical phase difference between the signal light and the local light which are inputted into the optical modulator in the optical transmitter is called an optical phase excursion. And, the difference between the carrier wave frequency of the signal light and the optical frequency of the local light is called an optical carrier wave frequency deviation. When an optical phase excursion exists, a signal with constellations rotated an optical phase excursion to the constellation shown in FIG. 1 is received as shown in FIG. 2 (a). Because a value of an optical phase excursion cannot be known in advance, a problem that wrong data is demodulated may arise if demodulating a symbol into a bit sequence by using symbol mapping shown in FIG. 1 as is.
If there is an optical carrier wave frequency deviation, the phase of the above-described beat signal becomes equal to the value obtained by adding the optical phase excursion to the product of the optical carrier wave frequency deviation and the receipt time. Therefore, as shown in FIG. 2 (b), a signal having constellations with the constellation shown in FIG. 1 rotating temporally is received. Because the phase of the beat signal changes temporally at that time, it is impossible to demodulate data from the phase of the beat signal using symbol mapping shown in FIG. 1.
Accordingly, in an optical phase modulation system, a function for compensating a phase excursion and a carrier wave frequency deviation is required which prevents a constellation from rotating by an optical phase excursion and an optical carrier wave frequency deviation. Hereinafter, a process for compensating a phase excursion and an optical carrier wave frequency deviation will be described, which is widely used in optical phase modulation systems.
Each diagram in FIG. 3 shows a configuration of an optical phase excursion and optical carrier wave frequency deviation compensation circuit (hereinafter, referred to as “a compensation circuit”). FIG. 3 (a) is called a feedforward-type and FIG. 3 (b) is called a feedback-type. Hereinafter, only the feedforward-type shown in FIG. 3 (a) will be described.
An input signal of a compensation circuit is branched into two signals. One of the branched input signals are inputted into a circuit which estimates a phase compensation amount. And the other of the branched input signals are multiplied by a complex number to apply a reverse rotation of the estimated amount of phase compensation, and outputted as a compensated signal.
The circuit which estimates the phase compensation amount includes a phase error detection unit 101, a filter unit 102 and a phase compensation amount calculation unit 103. The phase error detection unit 101 is a circuit which detects a change per unit time of an optical phase excursion, that is, a change in an optical phase excursion between two adjacent symbols. An M-th Power Algorithm (m-th power circuit) shown in FIG. 9 is widely used as such circuit.
The change in optical phase excursions between two adjacent symbols is equal to a product of an optical carrier wave angular frequency deviation and one symbol time (one symbol time is equal to a reciprocal of a symbol rate), and one symbol time is constant. Therefore, the phase error detection unit 101 is a circuit which calculates an optical carrier wave frequency deviation.
The output of the phase error detection unit 101 is sent to the filter unit 102, and a noise component is removed in the filter unit 102. The output of the filter unit 102 is sent to the phase compensation amount calculation unit 103, and the actual phase compensation amount, that is, the rotation of the constellation is calculated. Specifically, the phase compensation amount calculation unit 103 is equivalent to a circuit for holding by integrating.
Finally, a product of the input signal and a complex number (which is expressed as exp (−iφ), if the phase compensation amount is assumed to be φ) which applies a reverse-rotation of the phase compensation amount calculated by the phase compensation amount calculation unit 103 is outputted as an output signal.
FIG. 4 is a diagram showing an example of temporal change in a phase compensation amount estimated by the compensation circuit. A horizontal axis shows time and a vertical axis shows a phase compensation amount. The time will be a reciprocal of a symbol rate inputted to the compensation circuit. As shown in FIG. 4, the phase compensation amount temporally and continuously changes can be seen in this case.
As described above, in the optical communication system using the optical phase modulation system, the transmitted data is demodulated by preventing the constellation from rotating by using the optical phase excursion and optical carrier wave frequency deviation compensation circuit shown in FIG. 3. Because the circuit shown in FIG. 3 is generally executed by digital signal processing, the optical phase modulation system is also called an optical digital coherent system.
However, there is the following problem in realizing an ultra high-speed optical communication system using the above-mentioned system. When a noise component of light, for example, nonlinear noise like impulse noise, phase noise accompanied by a rapid phase change, and ASE noise are added to an input signal, as shown in α-1 of FIG. 5, an estimate value of the phase compensation amount may change discretely suddenly. ASE is an abbreviation of Amplified Spontaneous Emission. Therefore, even if the optical phase excursion and optical carrier wave frequency deviation compensation circuit shown in FIG. 3 is used, a rotation of a constellation by the rapid phase shifting (“a cycle slip”. Referred to as “a slip” hereinafter) cannot be prevented, and the symbol mapping changes (rotates). When this event once occurred, a wrong data will be demodulated because the slip occurrence cannot be confirmed in the above-mentioned system.
In order to solve the above-mentioned problem, a method called differential coding is generally used. The differential coding is a coding method with which a bit sequence is correlated to a transition between neighboring two symbols.
FIG. 6 shows an example of a constellation of QPSK to which the differential coding was applied. In the optical communication system using differential coding, as shown in FIG. 6, a bit sequence does not correspond to a symbol itself, but a bit sequence is mapped on a transition from a certain symbol to the same or another symbol. For example, in FIG. 6, a bit sequence 00 is allocated to the transition 1 in which a phase change between the neighboring symbols is +0 degree, a bit sequence 01 to the transition 2 with a phase change of +90 degrees, a bit sequence 11 to the transition 3 of +180 degrees, and a bit sequence 10 to the transition 4 of +270 degrees, respectively.
FIG. 7 is a block diagram showing a configuration of a compensation circuit in an optical communication system using differential coding. FIG. 7 is identical with the compensation circuit shown in FIG. 2 (a) except that a differential decoding unit 104 is added to the subsequent stage of the compensation circuit of FIG. 2(a). The differential decoding unit 104 transforms a transition between neighboring symbols into a predetermined bit sequence, as mentioned above. At that time, because a bit sequence correspond to a transition between neighboring symbols, transmitted data can be decoded correctly even if the constellation rotates.
As described above, if a slip occurred, transmitted data can be decoded correctly by applying differential coding.
As a related art, a technology for supplying a phase synchronization circuit with wide frequency range which can perform drawing-in operation, and a slip detecting device suitable for the phase synchronization circuit is disclosed in patent literature 1. This reduces access time until a signal of a specified recording position of a recording medium is read from an unlocked status.